1. Field of the Invention
This invention pertains to acoustic transmitting antennas which allow one to transmit within the sea acoustic signals which can penetrate into the undersea depths in order to determine the structure of this site, in particular to detect the possible presence of oil.
2. Dicussion of the Background
At present, exploration for oil at sea is conducted by immersing at shallow depth one or several acoustic sources and by using towed linear acoustic antennas which are dragged along behind a ship. These linear acoustic antennas are essentially made of a hollow cable in which hydrophones arranged along the length of this cable are enclosed. The towing ship drags these antennas at relatively slow speed, on the order of 5 knots, in order to accurately gather the signals.
The acoustic signals corresponding to the sources are directed toward the ocean bottom, to the interior of which they penetrate. They are then reflected, as in land exploration, by layers of the subsoil, then they enter the water again and are detected by the hydrophones of the acoustic linear antennas. The bottom of the sea behaves as an additional interface with respect to the interfaces between the different layers of the subsoil. The additional interface simply has more pronounced characteristics than the others, an obviously greater impedance discontinuity in particular.
Because of this considerable change of impedance, it is essentially the low frequency components of the shock wave which effectively penetrate into the undersea depths, however with relatively low efficiency. This phenomenon is identical during the recrossing of the waves between the ocean bottom and the water before reaching the hydrophones.
The overall efficiency thus being relatively low, one must use very powerful acoustic sources.
To date, the only sources which provide satisfaction are devices known by the name of air guns. These devices are not very satisfactory because they are heavy and cumbersome, and they require a compressed air power source from the towing ship.
In addition, the shock wave caused by operation of such a device propagates almost omnidirectionally. The result is that only the weak portion directed toward the sea floor is useful. In addition, the part directed toward the surface is reflected toward the latter and disturbs the useful portion of the impulse.
In order to overcome these disadvantages, different systems have been proposed. In particular, a U.S. Pat. No. 4,632,213 issued on Dec. 30, 1986 in the name of Samuel N. Domenico proposes that one enclose the air gun in a series of concentric tubes which will form the envelope of a paraboloid. These tubes are equipped with perforations through which one can inject the air. The mixture of water and air forms a reflecting discontinuity which has the shape of the paraboloid which encloses the tubes. One can thereby obtain the effect of a reflecting mirror which concentrates on the bottom of the sea the emitted shock wave. However, this device is quite complicated, which further aggravates the difficulties of its implementation. It has been determined in particular that this patent envisages only its use at a fixed station from a drilling platform. Indeed, if one anticipates towing such a device behind a ship, as is frequently the case for air guns used in the traditional manner, one will experience not only great difficulties of implementation, but the relative movement of the water with respect to the device will begin to considerably disturb the shape of the emulsified layer which forms the reflector, which would seriously harm the performance of the unit.